The present invention relates to a disposable injection syringe comprising an injection needle, a tubular cylinder with a free opening at a first end, an end part with a through hole for slidably accommodating the injection needle at an opening of a second end of the tubular cylinder opposite the free opening, a piston reciprocally accommodated in the cylinder, a piston rod attached to the piston and extending into the cylinder through the free opening of the cylinder, a coupling for interlocking the piston and the injection needle at least in the reverse direction of the injection piston stroke, wherein the coupling further comprises two coupling parts, a male coupling part which is attached to the piston and a female coupling part which is attached to the injection needle or vice versa, said coupling parts being spaced from each other during at least part of the injection stroke.
Known disposable syringes should be discarded to reduce the risk of getting or spreading an infection but it is often possible to reuse them. Such disposable syringes are often expensive to manufacture and therefore expensive to use, and in spite of safety regulations and injection control guidelines many persons and health authorities especially in poor countries violate these guidelines. In addition, many drug abusers unhesitatingly utilize the same contaminated syringe and needle without prior sterilisation, or several drug abusers share the same syringe and needle, thereby contributing to the spreading of blood borne, infectious diseases such as HIV virus (AIDS) and hepatitis B.
In order to prevent such reuses of disposable syringes, different disposable syringes have been developed, some of them being needleless systems as a safeguard against contamination and as a precaution against needle sticks for healthcare providers. However, even with needleless systems the reuse of syringes from patient to patient is widespread. Moreover, for the administration of many different medicaments, needleless systems cannot be used.
European Patent EP 0 639 992 B1 discloses a disposable prefilled syringe. This syringe has a cannula frictionally mounted inside the outlet connection piece or needle carrier. The inner edge of opposing piston rod legs are provided with barbs arranged to cooperate with ratchet means provided on the piston rod in order to interlock these two parts of the syringe thereby disenabling refilling of the syringe and preventing a further injection possibility. Following injection the piston is retracted bringing the needle along for accommodation into the interior of the cylinder.
Significant compressive forces are needed for performing the injection to secure the frictional engagement of ratchet and barbs. Also, some patients will be able to feel the stepwise engagement between the ratchets and the barbs during injection. The jerky motion of the piston when the barbs and ratchet snap together during the injection procedure is felt as an uneven and inconvenient experience by the patient. Furthermore, the many structural members of the syringe that need to co-operate nicely for producing a reliable interlocking contribute to increased manufacturing costs.
Another disposable injection syringe is known from British patent application 2,256,146-A. This syringe has a piston rod, which is connected to the piston by a single use connection. The connection breaks when the syringe has been completely discharged or if an attempt is made to reuse the syringe. The piston is designed with a hollow space for receiving and containing the needle when this has been detached from the syringe end after use.
This kind of syringe presents a high risk of pricking oneself when the piston rod is to be mounted over the needle tip for enclosing and hiding the needle.
European patent application EP 0 824 924 A1 discloses a disposable syringe provided with a needle holder for internal mounting of the needle hub of the needle. An arrowhead-like retainer rod on the piston is forced into a close fitting engagement with a complementarily shaped arrowhead-like retaining hole, to interlock with said hole in the direction of the injection stroke. Since the needle is situated inside the holder, both holder and needle is, after finishing of the injection procedure, for safety reasons retracted into the syringe chamber.
However, this known disposable syringe has several disadvantages. For one thing the needle holder is capable of being pressed inside the housing during preliminary mounting of the needle, thereby creating leakage between the holder and the outlet orifice of the syringe and rendering the syringe useless. Moreover the interlocking of rod and hole is not smooth and will unavoidable be felt by the patient.
U.S. Pat. No. 6,066,115 describes another disposable syringe of substantially similar design to the one described in EP 0 824 924. Both these known syringes require a relative large compression force in order to obtain coupling between the engagement parts.
Thus, there is a need for a syringe that overcomes the drawbacks of prior art syringes.
This need is now satisfied by the present invention.